In a boiling-water reactor (BWR), the intermediate range monitor performs the Campbell method to measure the signals output from the fission ionization chamber.
On the other hand, in a pressurized-water reactor (PWR), the intermediate range monitor has a gamma-ray compensated detector configured to detect the net current signal output from the proportional counters exposed to neutron.
A gamma-ray compensated neutron detecting apparatus is known. Like the gamma-ray compensated detector used in the PWR, this neutron detecting device has a neutron sensor sensitive to neutrons and gamma rays and a gamma-ray compensated unit sensitive to gamma rays only. In the neutron detecting apparatus, the signals output from the gamma-ray compensated unit in a time domain are used, correcting the output signals of the neutron sensor, thereby measuring the net value of each neutron signal.
To measure neutrons by using the current signals output from the gamma-ray compensated neutron detecting device, the difference between the output current of the neutron sensor sensitive to neutrons and the output current of the gamma-ray compensated unit is determined, thereby detecting the net value of the current generated by the neutron.
To measure neutrons by using the pulse signals output from the gamma-ray compensated neutron detecting device, the pulses output from the neutron sensor are counted and the pulses output from the gamma-ray compensated unit are also counted. The difference between the number of the output pulses of the neutron sensor and the number of output pulses of the gamma-ray compensated unit is determined, thereby detecting the net value of neutron signal. Alternatively, the output signal of the neutron sensor may be intercepted (i.e. gated) by the output signal of the gamma-ray compensated unit, thereby to count the signals generated only from the neutrons.
In order to measure the signal output from the gamma-ray compensated neutron detecting device using the differential current, or in order to count only the signals resulting from neutrons by gating the pulses output from the neutron sensor with the signals output from the gamma-ray compensated unit, it is necessary to ensure the coincidence between the output signals of the neutron sensor and the output signals of the gamma-ray compensated unit.
In the process of measuring the differential current, the current generated from gamma rays in the neutron sensor is cancelled out by the current generated from gamma rays in the gamma-ray compensated unit. Therefore, the neutron sensor and the gamma-ray compensated unit must respond in the same way to gamma rays. More precisely, the neutron sensor and the gamma-ray compensated unit must have same performance regarding a detector power supply and a detection signal amplifier.
In order to measure the signal output of the neutron detection apparatus from the difference between the number of pulses output from the neutron sensor and the number of pulses output from the gamma-ray compensated unit, the above-mentioned coincidence is not so strictly required. However, the neutron sensor and the gamma-ray compensated unit must have the same sensitivity to gamma rays. The circuits for measuring the signals output from the neutron detection apparatus should therefore have the same characteristics.